1. Field of Invention
The present invention is directed to modifications to log hogging apparatus. This is typically apparatus which breaks down logs and off-cuts of wood into smaller pieces, and which are commonly then used as a fuel. More specifically the present invention preferably finds use in hogging assemblies which use a screening process for separating comminuted product of sufficiently reduced size.
2. Description of the Related Art
‘Hog fuel’ is a combustible solid fuel often used in boilers and which is primarily made up of wood residue from sawmills, logging operations, and various wood off-cuts. Hog fuel typically varies from chipped wood as it commonly may comprise timber of a variety of sizes rather than substantially uniform chips of wood. Typically hog fuel may comprise pieces of wood typically 50 mm×50 mm×120 mm down to sawdust. It is commonly made from product which would otherwise be dumped, and may contain a significant amount of foreign materials such as dirt, and stones, depending on the source of the material and how well the material is screened prior to processing.
The machines used to prepare hog fuel are commonly known as ‘hoggers’ and may also be called reducers, depending on their construction. Quite a number of different sizes exist, and in most case rely on rotating discs or drums with chipping or cutting teeth or features. In many instances these are used in conjunction with a grate which screens reduced material suitable for hog fuel, from product which has not yet been sufficiently reduced. Most designs have a number of flaws or problems associated with them, which at least partially counts for the presence of a number of substantially different standard designs available on the market.
It is also common for many of these devices to be used for reducing a variety of materials. While the ideal starting material for hog fuel is woody material, many hoggers are starting to find application as a means for shredding green plant material. While woody material—particularly if at least partially dry—possesses some rigidity and weight which is desirable in any screening process (separating reduced from unreduced material), green and leafy material tends to block screens and exacerbates many of the problems to be described below.
Many hoggers have also found application reducing quite different types of materials, including removed asphalt roading, tyres, soft rock, soft metals and mild scrap steels, etc. Tyres in particular can represent a problem similar to green plant matter as the steel cords and belts can block screens rather than passing through. This again exacerbates types of problems such as described below.
In an ideal situation, reduced material will be removed as soon as possible from the chamber or vessel in which reducing is taking place. If it is not effectively removed, then already processed material will be reintroduced to the reducing blades or equipment, rather than fresh material. Additionally, failure to effectively remove reduced material also affects the throughput, as well as potentially binding the machine and placing increasing load on components and motors.
The most noticeable outcome of ineffective removal of reduced material is energy consumption. This affects not only the energy efficiency of the process (an important overall consideration) but also the size of the motors required to drive the various components. Poor energy efficiency is a common problem affecting most designs.
Even in ideal circumstances, the process of reducing timber product into smaller reduced pieces suitable for hog fuel consumes a significant amount of energy. Careful consideration needs to be given to the economy of the process to ensure that the energy expended does not exceed the resulting energy which can be obtained from what is essentially a low-grade fuel. In such a case it becomes uneconomic to produce the hog fuel. The implications of this can be more clearly seen if one considers that hog fuel is not only a useful and renewable energy source, but also that it is a waste recycling process. If the fuel is not made, not only is the hog fuel user affected, but the producer of the raw material is then faced with a waste disposal problem. Hence, there are a number of potentially realisable benefits from improving the overall efficiency of hogging processes producing hog fuel.
These benefits can also extend to other applications of hoggers, such as the reducing of other materials. In most cases the reduced material is able to be recycled for other applications, and thus the hogger acts as a useful recycling tool. However, recycling—unless there is government legislation requiring it—is typically a cost conscious exercise. Improving efficiency, to improve the economy of the process, can have a significant bearing on whether this type of recycling technique is used as opposed to landfill or dumping etc.
In a number of instances the specific design of the hogging apparatus has a bearing on energy efficiency, and there are a number of different types of design in use. The primary type of hogger design to be considered for the present invention is a tub-type hogger where a substantially cylindrical tub with open top contains a rotating horizontal disc (rotating about a vertical axis) near its bottom. The bottom of the tub typically comprises a grate which allows suitably reduced pieces of wooden material to fall therethrough. The rotating disc bears a plurality of teeth or projections which interact with bulk material which is loaded into the tub. However, it should be envisaged that the process of the present invention can be adapted and applied to other hogger designs.
There are a number of problems associated with the tub type of apparatus. For instance, there is typically a high wear and high energy usage associated with this particular design. As the disc is usually almost always totally immersed in various material, the blades are always being subjected to wear. Additionally a large amount of energy is also required to continue driving the disc in such conditions.
Associated with this type of apparatus are also problems with screening. Reduced material is produced by the interaction of the disc with the bulk material. Accordingly reduced material is in immediate proximity to the rotating disc, rather than the grate. Between this reduced material and the grate may be bulk material of too large a size to actually pass through the grate. Hence, bulk material within the tub may act as a barrier to the efficient removal of screen material as it is produced. Green plant matter or stringy material (such as the steel cords for tyres) significantly increase this problem, laying across screen apertures and effectively clogging them. As a consequence of screen clogging, already reduced material may be unnecessarily further reduced in size just because it is in the very vicinity of the rotating disc.
It has also being found that the degree of loading in this type of hogger affects its performance and efficiency. When the apparatus is under filled, it is found that a large amount of energy is wasted by the discs spinning with little interaction with material. In this condition the process may be more efficient in terms of removing material from within the tub, though it is also possible that quite large pieces may be expelled from the grate—e.g. relatively long lengths which may be of sufficiently small cross sections to pass through the grate in a substantially vertical orientation.
However, different types of problems occur when overfilling the tub with bulk material. When the optimum fill level is exceeded, then the disc may then act more as a mixer blade and either end up stirring the material, or arrive at a situation where the material close to the rotating disc can be over processed while material further away may be under processed. Such situations also give rise to earlier mentioned case where sufficiently reduced material is not quickly screened from the tub but remains in the system. As can be appreciated, such overfilling also increases the wear on the teeth as they are now in even more constant contact with bulk material.
As can be seen, poor screening efficiency can have a significant result on the efficiency of tub type hoggers, particularly if they lead to an over-full state. The result of actual or induced overfilling is typically a significant energy draw and wastage, as well as high component wear. It is also noted that such problems can also arise in other types of hogger design which rely on screening processes, though not normally to the same extent as they may occur in tub-type hoggers. Nevertheless there is still a need to look at improving screening processes in various hogger designs where problems of efficiency and component wear still arise.
It is therefore an object of the present invention to provide means to improve the screening efficiency of reduced material in hoggers.
It is also an object of the present invention to address the aforementioned problems.
At the very least it is an object of the present invention to provide the public with a useful choice.
Aspects of the present invention will be described by way of example only and with reference to the ensuing description.